Syntheses of oxygen-18 and deuterium labelled compounds are being developed for clinical studies on their metabolic fate and for internal standards in mass spectral assays. General synthetic reactions are being studied in an effort to provide key labelled intermediates in high yield. Syntheses of deuteromethylated catecholamine metabolites have been completed. Optically resolved 4-hydroxy-3-deuteromethoxyphenylethyl glycol (CD3-MHPG) has been prepared for a clinical study. Oxygen-18 labelled dopamine has been prepared by acid catalyzed exchange. Preparation of oxygen-18 substituted catechols has been achieved in high yield through chromium tricarbonyl complexed dihalobenzene intermediates.